Sirenians

Sirenia associated with group Tethytheria (hoofed mammals) - includes Demostylia & Proboscidea

• Split from these in Palaeocene (65-54mya)
• Returned to marine conditions in middle Eocene 

Evolution of Sirenaians:

• Probrastomus - quadrupedal swimming
• Protosiren - reduced hindlimbs
• Thrichechidae - reduced pelvis EXTANT MANATEE
• Halitherinae - dorsoventral undulations of enlarged tail
• Dugonginae - EXTANT DUGONG
• Hydrodamalis - EXTINCT STELLARS SEA COW

Overall change from tetrapod body form (paddles) to reduction of hindlimbs and pelvis and change in spinal articulation, allowing body to undulate and provide tail thrust

Largest Sirenian 8-9m = Stellar Sea Cow (Hydromalis gigos) - hunted to extinction by European explorers for food in late 1700s

Closests living relatives = Proboscidea (elephants) and Hyracoidea (hyrax)

Same features of skulls 

• Shape of arch under eye
• Presence of 'tusks'

Dugongs closer in skull form to terrestrial relatives than manatees

Dugongs

• Tail w/ flukes (whale-like)
• Solid upper lip
• Smooth but scared skin
• Body hair - short and rigid
• Visible ear opening
• Pointed ends to forelimbs, no nails
• Long, tusk-like incisor teeth

Manatees

• Rounded tail (beaver-like)
• Divided upper lip
• Pleated, rough skin w/ barnacles and algae
• Body hair - long and flexible
• Small, indistinct ear opening
• Rounded ends to forelimbs w/ nails
• No incisor teeth

Dugongs

• Asian-Indo-Pacific
• Critically endangered
• Shallow tropical areas

Manatees

• Esturine
• Americas and W. Africa

Stellar's Sea Cow

• Lived in margins of North Pacific

Large body = retains heat

• Thick, fatty insulating bludder layer

Bones are pachystotic and osteosclerotic 

• = counteract natural passive buoyance from blubber
• Osteroclerosis = local/general increase in mass/density of skeleton
  • Higher compactness of bone inner architecture
• Pachyostosis = big, thickened bones
  • Caused by hyperlplasy (enlargement) of bone outer membrane

Swim at slow pace and can dive <5m for 2-3mins

• Herbivorous - graze on seagrass beds

Horizontally distributed tails provide propulsion - allow manoeuvrability and precision grazing

• Swim w/ vertical body undulations = long, flexible backbone

Horizontal diaphragm - extends length of body cavity

Controls buoyancy and reduce pitch and roll

Seperates lungs and heart = thoracic cavity airtight

When pushed down = lungs inflate = buoyacy correction at top of mammal

• If individual rolls over - naturally correct itself with self-righting mechanism

Prefer water >2m deep = abundant vegetation - angiosperm feeders

Amazonian manatees - only freshwater species

Others marine but migrate into warmer brackish/fresh water

Minimum temperature requirements:

• Amazonian manatees - 25-30'C
• West Indian manatees - 20'C
  • Use hot water outfalls from coastal industry
• Dugongs - <19'C

Herbivorous folivores, supplemented by accidental ingestion of arthropods and (dugongs) buried bivalves

Metabolic rate of only 36% predicted by Kleiber's law for thier size

Need to ingest 5-10% body weight per day = forage for <8 hours per day

Long gut (<20m) and transit times of ~7 days to digest large amounts of low quality food

• Microbial activity and stomach enzymes digest food

Amazonian manatees -  fast <200 days during dry season

• Starvation through habitat loss major cause of mortality - directly or through infections of weakened animals

Complex social behaviours and communication = key to mating and maternal care

Female sirenians = polyoestrous

• Multiple oestrus cycles/ovulation throughout year but length of cycle unknown

Polygamous - 1:1 sex ratio

• Cow in oestrus attracts herds of males who follow her while she evades - male catches her mates with her

Life span of 50-70 years

• Sexual maturity between 3-10 years old

Reproductive output low - minimum period between births ~2.5 years but up to 7 years

• Gestation between 12-14 months
• Calves born in shallow water and remain with mum for ~18 months
• Mortality rates cause drastic impacts on populations